During processing of plastics, different types of additives often need to be added to meet different processing and application needs of material, and selection of additives is crucial in design of plastic modification formulas.
Additives are like various seasonings in cooking. Only by organically combining factors such as type, dosage, and matching method of seasonings (auxiliaries) with main dish (matrix resin) can delicious dishes be made. Principle is same when used in plastic modification, but it is not easy to design a high-performance, easy-to-process, and low-cost formula, and there are many factors that need to be considered.
Following is a collection of super practical plastic modification additives and 11 key points for selecting plastic modification additives.

 

Different types of additives have different target properties. Additives added to system can fully exert their expected functions and achieve target specified indicators. Among them, specified indicators are generally national standards, international standards, or performance requirements proposed by customers. Additives can be classified as follows according to performance of common target resin modifications.
1) Flame retardancy: inorganic phosphorus, organic halide, organic phosphide, organic silicon and nitride, etc.
2) Reinforcement: Mainly divided into two categories: fiber and whisker. Inorganic fibers such as glass fiber, asbestos fiber, carbon fiber, boron fiber, quartz fiber, graphite fiber and ceramic fiber, etc.; organic fibers such as PAN fiber, polyethylene fiber, PA fiber, PC fiber, PVC fiber and polyester fiber, etc., as well as metal whiskers such as aluminum, titanium, calcium, etc.
3) Toughening: high impact resins, such as CPE, MBS, ACR, SBS, ABS, EVA, modified petroleum resin (MPR), etc.; high impact rubber, such as ethylene propylene rubber (EPR), EPDM rubber (EPDM), nitrile rubber (NBR), styrene-butadiene rubber, natural rubber, butadiene rubber, chloroprene rubber, polyisobutylene and butadiene rubber, etc.
4) Wear resistance: graphite, MoS2, silica, etc.
5) Degradability: starch filling, degradation additives, etc.

Same additives also have different morphological distributions and different effects on modification effect. For powder additives, key influencing factor is their particle size.
1) The smaller particle size, the more beneficial it is to tensile strength and impact strength of filling material. For example, in terms of impact strength, for every 1 μm decrease in particle size of antimony trioxide, impact strength will double;
2) The smaller particle size of flame retardant, the better flame retardant effect. For example, adding 4% antimony trioxide with a particle size of 45 μm to ABS has same flame retardant effect as adding 1% antimony trioxide with a particle size of 0.03 μm;
3) The smaller particle size of colorant, the higher tinting power, the stronger hiding power, and the more uniform color. However, particle size of colorant is not as small as possible. There is a limit value, and limit value is different for different properties;
4) Taking carbon black as an example, the smaller particle size, the easier it is to form a network conductive path, and amount of carbon black added to achieve same conductive effect is reduced. However, like colorants, particle size also has a limit. If particle size is too small, it is easy to aggregate and difficult to disperse, and effect is not good.
5) For fibrous additives, the higher degree of fibrosis of additive, the better reinforcement effect. Among them, fibrosis degree of additive can be expressed by aspect ratio (L/D).

 

Good compatibility between resin and additives is an important basis for ensuring good performance of additives, ensuring good durability, extraction resistance, migration resistance and precipitation resistance during use. Good compatibility is a basic requirement, and even barrier formulations require additives to be distributed in layers in resin. Main methods to improve resin compatibility:
1) Addition of compatibility additives, such as surfactants, should be added in an amount that maximizes its effectiveness.
2) Surface treatment: Use compatibilizer or coupling agent as surface treatment agent to improve compatibility between resin and additives. Common coupling agents include: silanes, titanates and aluminates; compatibilizers include: maleic anhydride graft polymer corresponding to resin. Additives that require surface treatment include inorganic additives and fiber additives.

Appropriate addition of additives can not only improve target resin to appropriate performance, but also keep costs low. There are different requirements for different additive amounts:
1) Flame retardants, toughening agents, magnetic powders, barrier agents, etc., although the more added the better from a performance perspective, cost must also be determined;
2) Conductive additives generally form a circuit path;
3) Use antistatic agent to form a charge-discharging layer on the surface;
4) Use coupling agent to form surface coating.

Addition of additives cannot lead to deterioration of resin's properties. For example, lead- and copper-containing additives cannot be added to PPS, and antimony trioxide cannot be used in PC. These can lead to depolymerization; at the same time, acidity and alkalinity of additive should be consistent with acidity and alkalinity of resin, otherwise the two will react.

In a formula, in order to serve multiple purposes at the same time, a variety of different additives need to be added. Interaction of additives is very complex. Main overview is:
1) Independent of each other: no influence;
2) Synergy: Multiple additives with heavy formulas promote each other, making the overall effect higher than average of a single additive;
3) Mutual elimination: When two or more additives are added together, effect is lower than average value when added alone. For example: in anti-aging plastic formulas, thioether auxiliary antioxidants and HALS light stabilizers should not be used at the same time, because acidic components generated by thioethers inhibit light stabilization effect of HALS.

Uniform mixing is basic requirement for plastic formula modification, and it also requires additives to be evenly distributed in resin. Uneven distribution of additives not only fails to improve performance of original resin, but may also cause performance to be worse than that of pure resin due to uneven distribution of fillers. For example, use of fillers will cause uneven dispersion of mechanical properties and processing properties of material. Not only will mechanical properties be greatly reduced, but processing properties will also be affected.
Common ways to improve uniform mixing of additives:
1) Reasonably sequence added fillers, for example: during coupling process, fillers are advanced, then coupling agent is added after heating and dehydration;
2) Components should be added to system in primary and secondary order. Large quantities of fillers can be added in multiple batches to facilitate uniform mixing. Coupling agent treatment generally needs to be sprayed in three times to achieve even dispersion and good coupling effect.

 

Fluidity of plastics is main aspect that affects processing performance of plastics, and is also an important basis for choosing which plastic processing method to use.
Generally speaking, addition of inorganic fillers will lead to a decrease in flow performance, and flow modifications need to be added for optimization. Common organic additives include: decabromodiphenyl ether and tetrabromobisphenol A.

Reduce processing temperature of resin, and processing temperature does not exceed decomposition temperature of additives to ensure stable existence of additives and reflect modification performance; add a certain amount of antioxidant additives to prevent thermal decomposition from occurring and affecting performance of resin.
Generally speaking, most organic dyes have a low decomposition temperature and are not suitable for high-temperature processing engineering plastics; spices have a low decomposition temperature, generally below 150℃, so only low-processing-temperature resins such as EVA can be used as carriers.

Environmental protection of formula includes: harmless to human body, equipment, environment and users. Human Hygiene - Resin and selected additives should be absolutely non-toxic, or their content should be controlled within specified range. Environmental pollution - selected components must not pollute environment.

The lower price of plastic modification formula, the better. During implementation process, use of additives should be based on following principles:
1) Prioritize use of existing raw materials: no need to purchase, stable sources, clear performance;
2) Prioritize use of domestic raw materials: price fluctuations are less affected by foreign exchange and trade policies;
3) Prioritize use of nearby raw materials: reduce cost of inventory;
4) Prioritize use of general-purpose raw materials: they come from a wide range of sources and have relatively stable performance.